The investigators are using the vitamin B12 receptor to target the delivery of cytotoxic anticancer drugs to rapidly dividing cells in leukemia and solid tumor. Drug-B12 bioconjugates are synthesized by attaching various cytotoxic warheads to the cobalt atom of cobalamin or a structurally-related corrinoid. The active cytotoxic drug is released spontaneously inside of immature leukemia cells, but no toxicity is observed when receptor-mediated endocytosis is blocked by presaturation of the receptors with vitamin B12. In solid tumors and all other tissues, the pro-drug is non-toxic until it is activated by photolysis with tissue-penetrating red light, or with sonolysis by focused ultrasound. Our results in vitro show this "Trojan Horse" strategy for targeted drug delivery to be a very effective method to increase the therapeutic index of existing anticancer drugs. For the immediate budget period, we will prepare bioconjugates of 7 cytotoxic anticancer drugs, including the chlorambucil bioconjugate that we have already synthesized. Bioconjugates of authentic B12 (cobalamin), as well as a close analog of B12 that has a more favorable absorption spectrum in the red region will be used. The stability of these bioconjugates, as well as their decomposition under photolytic and sonolytic conditions will be studied, and their toxicity against NCI human cancer cell lines in vitro will be evaluated. The bioconjugates that show favorable activity in vitro will be tested against tumors in mice. Fluorescent analogs of cobalamine will be used to study the cellular targeting and intracellular trafficking of cobalamin in tumor cells. The specific program goals are chemical synthesis, followed by in vitro and in vivo biological testing.